Originally the plan was to branch Mesa 9.1 last Wednesday while putting out the gold 9.1 release on 22 February. However, Ian Romanick, the Intel release manager of Mesa, shared on Saturday that things had gotten behind schedule. Romanick now plans to branch Mesa tomorrow (Monday) morning via this mailing list message.
Mesa 9.1 features OpenGL improvements to core Mesa along with Gallium3D, various improvements to the different Gallium3D state trackers (including OpenCL), performance improvements for the major drivers, new hardware support, and other new/improved functionality. See the Phoronix articles on Mesa over the past six months for more details; summary articles will be forthcoming as the official release approaches by the end of next month.